Panel has become unreliable and believe it is the panel not the wiring.  I have 12 proximity sensors, 12 smoke detectors and 2 heat detectors.  I also have 3 remote access keypads with simple LCD display.  I would like to add another 6 sensors.  My questions are:  Should I reuse the existing wiring? (its an old house and took a great deal of effort to fit professionally). Which system would provide the most intuitive fault finding?  Which system / manufacturer offers the most scope for the future? Can I reuse all the existing detectors? Would ideally like to be able to look in from a pc or iPad to see status?     Any comments or suggestions gratefully received.  

Most modern systems - Honeywell Galaxy, Texecom, Etc.  have many zones capability with double balanced circuits.  This means that you can detect open and short circuits on each sensor and get a detailed message from the panel to tell you what's going on.  You can use existing wiring and sensors. For existing sensors, you just add a couple of resistors to the contacts to make them double balanced.  Each active sensor only requires 3 cores and passive sensors 2 cores.  

You can also add zone expansion using RIO's (remote I/O) for situations where you have an existing wire going to a point (existing sensor say) and you want to expand to another 8 sensors in that area.

I use the Honeywell Dimension 96 (providing up to 96 detection circuits and 48 outputs) in my own home with two active 115db backlit Texecom sirens, multiple keypads, a power RIO and regular RIO. I have the E080 Ethernet module and SelfMon automation module, so have phone/smart device apps and can control the heating, garage doors, gates and lighting in combination with Home Assistant.  The Dimension has a nice big reliable linear power supply - I've never seen one of those go faulty - cannot say the same for Texecom's PSU's which were plagued with issues.

Disclosure - I run the SelfMon self monitoring platform.

As Stu really doubt there is a wiring issue, get your installer to test and reuse it. Always use a wire if you can. 

StuartSelfMon. many thanks for your reply.  Most helpful.  Given me a good direction to explore.  I believe strongly in wires as it is old house and wireless can be flaky. will report back when I work out what I need to buy.